1. Field of the Invention
The invention relates to hardness testers, and more specifically to penetration hardness testers that can perform more than one type of penetration hardness test.
2. Description of Related Art
Penetration hardness testers are well-known in the art, and generally include a diamond or ball tipped penetrator and means to apply minor or major loads of predetermined magnitudes through the penetrator to a test specimen in successive load cycles. The hardness of the surface being tested produces results such as a Rockwell number or Brinell number. The hardness is related to the depth of penetration of the penetrator into the surface when a selectable value of compressive force is applied to the penetrator. Optical measurement of diagonal length of an indentation is performed for Vickers and Knoop tests, for example.
In prior art Rockwell type hardness testers, the force that is exerted on the penetrator is produced by gravity acting on weights, and this in turn is transferred by mechanical means to the penetrator. The depth of penetration is generally directly measured from the tool and generally displayed on a dial indicator, digital display or other display apparatus. Prior art apparatus requires gravity acting on weights, and the measurement of the tool movement through mechanical assemblies is subject to impreciseness as the tester is repeatedly utilized subjecting the apparatus to wear through repeated mechanical movement.
The use of deadweight testers and their mechanical impreciseness over time has led to the use of a load cell as part of the means to measure the application of force to the test specimen. An example of a system employing a load cell is found in U.S. Pat. No. 4,535,623 entitled Material Hardness Testing Apparatus by Paul Gilberto, a patent assigned to a predecessor of the assignee of the present application and now owned by the instant assignee. In the '623 patent, a load cell is located adjacent the penetrator, and deadweights are avoided in conducting the hardness tests. A mechanical threaded advancing means is employed to apply the load to the test specimen, and the load on the load cell is related to the force on the test specimen. The mechanical action in the '623 patent for applying force by the tester, by its very nature, will, over time cause impreciseness because of the relative movement of the threaded screw and its driven elements. Such inaccuracies can become significant in the measurement process as the underlying measurements are used as a basis for many determinations thereafter.
The use of feedback control closed loop systems can lessen the impreciseness which is attendant to materials hardness tests. U.S. Pat. No. 4,435,976 describes the use of a load cell to determine the forces applied during Brinell tests and employs a feedback loop to automatically compensate factors which affect the accuracy of the measurements, such factors being temperature and friction. The apparatus in the '976 patent utilizes a mechanical bearing connected between the indenter and the load cell, which mechanical bearing, itself, can cause inaccuracies in the measurement process because of its repeated mechanical movement and the wearing of the bearing.
The indenter will penetrate to some depth or displacement in the test specimen. A measurement is made of the displacement, and in prior art penetration hardness testers, there are moving mechanical parts which move relative to each other located between the actual displacement and measured displacement. Such relative mechanical movement can contribute to sources of friction or lost (non-recoverable) displacement between the point of displacement measurement and the test specimens so as to impair the repeated accuracy of the hardness test.
All known bottom-referencing type hardness testing machines, both using load cell or deadweight style, employ an elevating screw to accommodate different specimen sizes. The mechanical forces employed in the elevating screw also can contribute to degradation of displacement measurement accuracy because of the possibility of additional deflection loss which can contribute to the inaccuracy of the displacement measurement. U.S. Pat. No. 5,616,857 to Merck et al. and assigned to the instant assignee (the teachings of which are incorporated herein by reference), for example, teaches the use of different sized platforms for mounting and supporting specimens in lieu of an elevating screw.
The use of load cells has increased the accuracy of hardness testers. However, load cells are made less accurate by increasing the amount of dead weight hanging therefrom. Weight and/or structure below the load cell creates dynamic forces that decrease the accuracy of the load cell readings.
Another drawback to conventional microhardness testers is that they cannot be easily reconfigured from being able to perform one type of hardness test to being able to perform another. This detraction can be particularly inconvenient when it is desired to perform more than one test on the same specimen. An interesting non-microhardness tester is taught in U.S. Pat. No. 5,177,999 to Tobolski et al., assigned to a predecessor in interest to the instant assignee and now owned by the instant assignee (the teachings of which are incorporated herein by reference). Tobolski et al. teach the provision of several indenters mounted on a rotatable turret. The turret is brought down to the specimen, and the spring-loaded indenter provides an indentation. If a second test is desired, the turret is rotated so that a different indenter is positioned over the specimen. A dead weight assembly provides the load in the Tobolski patent. Because the indenters are spring loaded, they move with respect to the turret. As a result, additional moving parts render the load applications and measurements thereof less accurate, in that there are greater opportunities for dead mechanical movement in the system. Moreover, one is limited in the number of different indenters one may use by the availability of mounting spots on the turret.